Television receiver



Dec. 2, 1941. J. VAN DER MARK 2,265,090

TELEVIS ION RECEIVER Filed March 2, 1958 lNVENTOR JAN (/4 A505 MAR/ATTORNEY Patented Dec. 2, 1941 UNITED STATES PATENT OFFICE TELEVISIONRECEIVER poration of Delaware Application March 2, 1938, Serial No.193,403 In Germany March 8, 1937 (C1. PIS-7.7)

1 Claim.

This invention relates to a television receiving circuit andparticularly to the part thereof which serves for synchronizing thescanning movement in the receiver with the transmitter to be received.

For the purpose of synchronizing the transmitter and the receiver it haspreviously been proposed to transmit jointly with the picture currentsline and frame synchronizing impulses, the first-mentioned impulsesbeing produced in the transmitter at the end of the scanning of apicture line, the latter at the end of the scanning of an entire pictureand usually the duration of the picture impulses including the scanningtime of a few picture lines.

In a television receiver arranged for the reception of the synchronizingimpulses transmitted in this manner jointly with the picture currents,the line and picture impulses respectively are supplied to a devicewhich controls the scanning movement in a direction parallel andperpendicular respectively to the picture lines.

If a cathode ray tube is used as the reproduction device each of thescanning control devices will generally comprise a generator, referredto hereinafter as a sawtooth generator, for the production of a sawtoothshaped current or voltage and, in addition, one or more deflecting meansinterconnected via a coupling element that may contain an amplifier.

In order that the curved shape of the current or the voltage produced bythe sawtooth generator may be retained, the coupling element whichconnects the corresponding sawtooth generator to the deflecting means isproportioned in practice in each of the scanning devices in such mannerthat the basic frequency and all the higher frequencies are amplifiedwithout distortion. The basic frequency is to be understood to mean herethe frequency of the current or the voltage produced by the sawtoothgenerator. This means that the lowest frequency that is let through bythe coupling element that forms part of the device referred tohereinafter as the horizontal scanning device that controls the scanningmovement in a'direction parallel to the picture lines is the linefrequency i. e. the number of lines scanned per second. The horizontalscanning device is controlled by the received line impulses, a few lineimpulses being, however, suppressed during the occurrence of a pictureimpulse at the end of the scanning of each entire picture.

The lowest frequency that is let through by the coupling element thatforms part of the device referred to hereinafter as the verticalscanning device that controls the scanning movement in a directionperpendicular to the picture lines, is the picture frequency i. e. thenumber of pictures scanned per second. The vertical scanning device iscontrolled by the received picture impulses.

It has been found that in the use of a television receiving circuit ofthe kind above indicated the first picture lines of the reproducedpicture are often laterally displaced.

According to the invention, this disadvantage is obviated by thecoupling element that forms part of the horizontal scanning device beingso dimensioned that the time constant of this element is of the order ofmagnitude of the periodic time of the sawtooth shaped current or voltageproduced in the vertical scanning device.

In order that the invention may be clearly understood and readilycarried into effect, it will now be described more fully with referenceto the accompanying drawing, in which Fig. 1 shows one form ofconstruction of a television receiver according to the invention,

Fig. 2 is one form of construction of a coupling element that may beused in the circuit according to the invention,

The operation of the circuit according to the invention will bedescribed in greater detail by reference to the curves of Figs. 3a, 3b,3c and 311.

In the television receiver according to Fig. 1, the modulated highfrequency oscillations received in an antenna I are supplied to a mixingtube 2 which is also devised for the production of local oscillations oris coupled to a local oscillator 3. The oscillations occurring in theoutput circuit of the mixng tube 2 are supplied via an intermediatefrequency amplifier 4 to a detector 5 and the picture currents andsynchronizing impulses obtained after detection are fed, in some casesvia a low frequency amplifier 6, to the control electrode of a cathoderay tube 1, the received picture currents controlling the intensity ofthe cathode ray beam generated in the tube '1 and the cathode ray beambeing suppressed by the received synchronizing impulses, or as iscommonly termed, the return line is eliminated.

In order that the movement of the cathode ray beam in the tube 1 may beperformed in synchronism with the scanning movement in the transmitterto be received, the synchronizing impulses and picture currents obtainedafter detection are fed, via a device 8 for separating picture currentsand synchronizing impulses, to the scanning devices. If desired, theconductor which connects the device 8 to the vertical scanning devicemay include a filter 9 which lets through the vertical synchronizingimpulses only.

The horizontal scanning device comprises a generator ID for theproduction of a sawtooth shaped voltage the frequency of which is equalto the line frequency. The vertical scanning device comprises a sawtoothgenerator H which produces a voltage the frequency of which equals thepicture frequency. We will not enter into details of the arrangement ofsuch a sawtooth generator since the generator per se does not fallwithin the scope of the invention. The scanning devices comprise inaddition a pair of deflecting plates M and I respectively which areconnected via a coupling element l2 and 3 respectively to the associatedsawtooth generator Ill and II respectively. evident that magneticdeflection may be used, if desired.

The coupling element shown in Fig. 2, which is frequently used in thetelevision receiver described, comprises a resistance 2|! which hassupplied to it the sawtooth shaped voltage produced by the generator Itor H and has connected in parallel with it the series combination of acondenser 2| and a resistance 22. The sawtooth shaped alternatingvoltage which is set up across the resistance 22 is supplied, in somecases via an amplifier (not shown), to the deflecting plates M or IE. Itis also possible for the voltage produced in the generator IE or H to besupplied to the resistance 20 via one or more amplifier valves.

In the well known television receivers whose construction correspondsessentially to the diagrammatic circuit shown in Fig. 1 it is commonpractice to dimension th time constant of the coupling element l2 insuch manner that the line frequency produced by the generator IO and allthe higher frequencies are let through, whereas, the time constant ofthe coupling element I3 is such that this coupling element lets throughthe picture frequency and all the higher frequencies.

As already observed, such an adjustment presents a disadvantage in thatthe first scanning lines of the reproduced picture are laterallydisplaced. The invention is based on the recognition of the fact thatthis displacement is due to the suppression of the line impulses duringthe reception of a picture impulse, since when the line impulses aresuppressed, the sawtooth generator It! will produce a sawtooth shapedvoltage whose frequency is governed by the natural frequency of thegenerator and differs slightly from the frequency that is set up duringthe reception of line impulses. The adjustment of the sawtooth generatoris such that in the absence of line impulses the frequency of theoscillations to be generated and also the amplitude of theseoscillations increases, which may be readily seen when it is realizedthat the operation of a sawtooth generator is frequently based on asudden discharge or charge of a condenser charged or discharged via aresistance and this discharge or charge ensues under the influence of areceived line impulse. In the absence of the line impulses It is,however, clearly the condenser charge or discharge does not ensue untilat a later time and in this case the condenser is charged to a highervoltage or discharged to a lower voltage. Thus, at the end of thescanning of each entire picture the sawtooth generator ID of thehorizontal scanning device produces a voltage which as a function oftime has a variation of the kind shown in Fig. 3a of the drawing, inwhich the curve shown represents the voltage set up by the generator |0during the last two scanning lines, during the reception of a framesynchronizing impulse and during the first eight lines of the followingpicture.

When this voltage is supplied to the resistance 20 of a coupling elementof the kind shown in Fig. 2 which lets through the line frequency andall the higher frequencies, it is found that across the resistance 22 ofthe coupling element is set up a voltage whose variation as a functionof time is shown in Fig. 3b and results in the above described lateraldisplacement of the picture lines. Across the condenser 2| is set up avoltage whose variation is shown in Fig. 30. This phenomenon is due tothe fact that the voltage supplied to the resistance 20 contains a lowfrequency component whose variation is shown in dotted lines in Fig. 3dand whose basic frequency corresponds to the picture frequency.

According to the invention, the time constant of the coupling element isdimensioned in such manner that even the picture frequency is letthrough without distortion. In this case the frequency referred tooccurs across the resistance 22 so that the voltage across the condenser2| is constant and the lateral displacement is avoided. The voltageoccuring at the output terminals of the coupling element is shown infull lines in Fig. 3d.

Referring to Fig. 3b and 30, it has been assumed for simplicity that thecharge and discharge of the condenser 2| has a linear course. In fact,however, the course of the Voltage across the condenser 2| differsslightly from the linear shape.

What I claim is:

In a television receiver wherein video, line synchronizing, and framesynchronizing signals are received, means for separating said signals, acathode ray reproducing tube for reproducing the televised image, saidtube including means for generating and accelerating an electron beam,means for deflecting the electron stream within said tube, an oscillatorfor providing a sawtooth wave substantially at the line frequency, meansfor impressing said line and frame synchronizing signals onto said linesawtooth wave generator the amplitude of said wave being greater duringthe receipt of frame synchronizing signals, and a circuit comprising apair of resistive members joined through a condenser coupling saidsawtooth wave generator to the line deflecting means within said cathoderay tube, said coupling circuit having a time constant substantiallyequal to the period of the frame synchronizing signals.

JAN VAN DER MARK.

